Just keep reading about borders and how to measure for them . Yes, you measure from the centers of the quilt top and not the edges. This reduces and eliminates the "wavy border" . Relax and have fun. Trial and error...we all learn. Peruse the picture section to view how others interpreted their borders. There is a trend to NOT add a final border esp on pieced tops vs ones using panels. It is your creativity that is where the fun begins. She taught you the basics. Now you tke over on the creativity and personal preference.
sandy